Surgical prostheses

ABSTRACT

A prosthesis arranged to be coupled to a bone. The prosthesis comprises a substrate ( 2 ) having a surface ( 6 ). The surface ( 6 ) of the substrate ( 2 ) has a first area and a second area, the first area being treated such that osseointegration is promoted more than in the second area. The interface ( 12 ) between the first and second areas forms an alignment mark to assist alignment of the prosthesis relative to a bone. The prosthesis is arranged to be at least partially inserted into a bone cavity such that the position of the alignment mark relative to the bone cavity is indicative of the angle of insertion of the prosthesis or the alignment mark provides a position reference for determining the implanted position of the prosthesis in the cavity. A method of manufacturing the prosthesis and a method of implanting the prosthesis are also provided.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. application Ser. No.13/266,013, filed Apr. 9, 2010, which is a National Stage 35 U.S.C. 371of International Patent Application PCT/GB2010/000718, filed Apr. 9,2010.

The present invention relates to surgical prostheses. The presentinvention is particular suited to a rotationally symmetrical surgicalprosthesis such as an acetabular cup.

It is well known to repair bone joints damaged through disease or injuryby implanting prosthetic components to replace part or all of thenatural bone joint. For example, surgical reconstruction of a hip jointmay require a femoral prosthetic component implanted at the end of thefemur to replace the natural femoral head with a prosthetic bearing headand a prosthetic acetabular cup implanted within a reamed acetabularcavity or the natural acetabulum to receive the prosthetic bearing head.

There are a range of different fixation techniques known for securingprostheses to the surface of bones, or within bone cavities.Furthermore, these fixation techniques may be used in combination.Commonly, mechanical fixation is provided by securing the prosthesiswith screws, pegs, wires or similar fasteners extending from theprosthesis into the bone. It is also known to provide a coating to asurface of a prosthesis which when implanted is in contact with thebone, or in close proximity to the bone, where the coating is chosen topromote osseointegration. Osseointegration is the direct structural andfunctional connection between living bone and the surface of aprosthesis. Osseointegration may either result from mechanical retentionwhereby bone ingrowth into surface features of a prosthesis, inparticular a metal prosthesis, secures the prosthesis to the bone, orbioactive retention whereby the implant is coated with a bioactivematerial which stimulates bone formation leading to a chemical bond inwhich the implant is ankylosed with the bone.

Osseointegration by mechanical fixation occurs for a number of metalscommonly used within implantable prostheses, such as titanium andtitanium alloys. It can be encouraged by the provision of topologicalfeatures like vents, slots and dimples upon the surface of theprosthesis in contact with the bone. There is no chemical retention ofthe prosthesis and the retention is dependent upon the surface area ofthe prosthesis.

Osseointegration by mechanical fixation may be encouraged by treating asurface portion of a prosthesis to increase its surface area, forexample etching the surface. Alternatively, it is known to apply aporous coating to the metallic substrate such that bone ingrowth intothe pores forms a firm bond between the prosthesis and the bone. Theporous coating may consist of a plurality of small discrete particles ofa metallic material bonded together at their points of contact to definea plurality of connected interstitial pores in the coating. Such acoating material, and a method of forming the coating, is described inU.S. Pat. No. 3,855,638. Preferably, the particles are of the samemetallic material as the substrate. The coating may be formed byapplying an adhesive to the portions of the substrate to be coated andapplying the particles to the adhesive. Alternatively, a slurry ofmetallic powder suspended in an aqueous solution may be formed andapplied to the substrate. The prosthesis is then sintered to remove theadhesive or aqueous solution and to fuse the particles together and tothe substrate. Such a porous coating is commercially available fromDePuy Orthopaedics, Inc under the name Porocoat.

Bioactive osseointegration occurs when the coating stimulates boneformation. A suitable coating material is hydroxyapatite (HA, also knownas hydroxylapatite). Hydroxyapatite is a naturally occurring mineralform of calcium apatite which forms up to seventy percent of naturalbone. Hydroxyapatite is commonly used as a filler to replace amputatedbone or as a prosthesis coating to promote osseointegration.

Correct alignment of an implanted prosthesis relative to the naturalbone, and in particular to any reamed cavity within the bone arranged toreceive the prosthesis, is essential to ensure a strong bond to the boneand to achieve correct mobility of the reassembled joint. For instance,for an acetabular cup, a cavity is formed in the acetabulum (or ifappropriate the natural acetabular cavity may be used) shaped to receivethe cup (which generally has a hemispherical outer surface). The cup isintended to be positioned eccentrically within the cavity such that aportion of the cup protrudes above the rim of the cavity. However,inexperienced surgeons may mistakenly believe that the cup is intendedto be seated and secured in position flush with the acetabulum rim inorder to replicate the cups natural orientation. This is incorrect andcan restrict the movement of the hip.

A second problem with conventional acetabular cup placement results fromdifficulty in positioning the cup owing to the surgeons viewing angle.To position an acetabular cup, typically the cup is inserted at aninclination of 40° relative to the patient's longitudinal axis. It isthen typically necessary to apply 20° of anteversion (rotation about thepatient's longitudinal axis) to assume a correct anatomic position.However, if the surgeon is viewing the patient on an anterior-posteriorplane then when the operative anteversion is applied the inclinationangle appears to increase. This compound angle effect could cause aninexperienced surgeon to compensate by reducing the inclination.However, this results in incorrect cup implantation, which increases thewear rate of the cup.

A third problem is that for conventional acetabular cups, when correctlyplaced in the cavity portions of the cup protruding from the cavity maycomprise rough surfaces due to surface treatment of the substrate topromote osseointegration. The rough surfaces may abrade surrounding softtissues.

It is an object of embodiments of the present invention to obviate ormitigate one or more of the problems associated with the prior art,whether identified herein or elsewhere. In particular it is an object ofembodiments of the present invention to provide a prosthesis which aidsthe surgeon in correctly positioning the implanted prosthesis byreference to local bone landmarks.

According to a first aspect of the present invention there is provided aprosthesis arranged to be coupled to a bone, the prosthesis comprising asubstrate having a surface: wherein the surface of the substrate has afirst area and a second area, the first area being treated such thatosseointegration is promoted more than in the second area; wherein theinterface between the first and second areas forms an alignment mark toassist alignment of the prosthesis relative to a bone; and wherein theprosthesis is arranged to be at least partially inserted into a bonecavity such that the position of the alignment mark relative to the bonecavity is indicative of the angle of insertion of the prosthesis or thealignment mark provides a position reference for determining theimplanted position of the prosthesis in the cavity.

An advantage of the first aspect of the present invention is that thealignment mark assists a surgeon in aligning the prosthesis relative tolocal bone landmarks. For instance, for embodiments of the presentinvention relating to acetabular cups, the alignment mark may indicate acorrect alignment of the cup relative to the acetabular cavity. Morespecifically, the alignment mark may advantageously identify to thesurgeon the correct proportion of the cup to protrude from the bonecavity. This reduces the risk of a cup incorrectly being inserted flushwith the acetabular rim or the inclination angle being set incorrectlydue to the effect of compound angles from the surgeon's viewing angle.The surface treatment may comprise applying a coating to the substrateto promote osseointegration. Alternatively, the surface treatment maycomprise grit blasting the surface or otherwise treating the surface toremove material to increase the porosity or surface area of thesubstrate. The surface treatment may only be applied to the first area.Alternatively, the first area and the second area may both be treated,but the amount of treatment or the type of treatment may vary betweenthe two areas. For instance, the depth of a coating material may begreater in the first area.

The position of the alignment mark relative to the bone cavity may beindicative of the portion of the prosthesis extending from the cavity.

The prosthesis may comprise a coating applied to the surface of thesubstrate in at least the first area to promote osseointegration, thesecond area comprising an interruption in the coating to form thealignment mark

According to an embodiment of the present invention there is provided aprosthesis arranged to be coupled to a bone, the prosthesis comprising asubstrate and a coating applied to a surface of the substrate to promoteosseointegration; wherein at least one interruption in the coating isprovided forming an alignment mark to assist alignment of the prosthesisrelative to a bone; and wherein the prosthesis is arranged to be atleast partially inserted into a bone cavity such that the position ofthe interruption in the coating relative to the bone cavity isindicative of the angle of insertion of the prosthesis or the portion ofthe prosthesis extending from the cavity.

At least a portion of the coating may be arranged in use to be incontact with bone tissue. The coating may comprise a porous coatinghaving a greater surface area than the underlying substrate.Alternatively, the coating may comprise a bioactive material to promotebone growth. The interruption in the coating may be detectable as adifference in roughness of the prosthesis surface and/or a stepdiscontinuity in the prosthesis surface.

The prosthesis may be rotationally symmetrical about a first axisextending into the cavity. In particular, the prosthesis may comprise acup arranged to be inserted into a bone cavity to form a socketcomponent of a prosthetic ball and socket joint. The interruption in thecoating may extend across a portion of a convex surface of the cup froma rim of the cup to a dividing line extending across the convex cupsurface, the dividing line extending relative to the rim of the cup at apredetermined angle such that the interruption in the coatingcorresponds to a portion of the cup which is intended to protrude from abone cavity when the cup is tilted within the bone cavity.Alternatively, the interruption in the coating may be formed along atleast one line across the convex surface of the cup extending relativeto the rim of the cup at a predetermined angle.

According to a second aspect of the present invention there is provideda method of manufacturing a prosthesis comprising: providing asubstrate; identifying a first area and a second area; and treating thefirst area such that osseointegration is promoted more than in thesecond area, the interface between the first and second areas forming analignment mark to assist alignment of the prosthesis relative to a bone;wherein the prosthesis is arranged to be at least partially insertedinto a bone cavity such that the position of the alignment mark relativeto the bone cavity is indicative of the angle of insertion of theprosthesis or the alignment mark provides a position reference fordetermining the implanted position of the prosthesis in the cavity.

Said step of treating the first area may comprise: masking the secondarea of the surface of the substrate; and applying a coating to promoteosseointegration to the surface of the substrate in the first area suchthat the coating adheres to the non-masked first area of the substrateand the masked second area of the substrate comprises an interruption inthe coating forming the alignment mark.

According to an embodiment of the present invention there is provided amethod of manufacturing a prosthesis comprising: providing a substrate;masking a portion of a surface of a substrate; and applying a coating topromote osseointegration to the surface of the substrate such that thecoating adheres to the non-masked portions of the substrate and themasked portion comprises an interruption in the coating forming analignment mark to assist alignment of the prosthesis relative to a bone;wherein the prosthesis is arranged to be at least partially insertedinto a bone cavity such that the position of the interruption in thecoating relative to the bone cavity is indicative of the angle ofinsertion of the prosthesis or the portion of the prosthesis extendingfrom the cavity.

According to a third aspect of the present invention there is provided amethod of implanting a prosthesis comprising: inserting a prosthesisinto a bone cavity, the prosthesis comprising a substrate having asurface, wherein the surface of the substrate has a first area and asecond area, the first area being treated such that osseointegration ispromoted more than in the second area, and wherein the interface betweenthe first and second areas forms an alignment mark to assist alignmentof the prosthesis relative to a bone; and aligning the alignment markrelative to a local bone feature by rotating the prosthesis within thebone cavity such that the position of the alignment mark relative to thebone cavity is indicative of the angle of insertion of the prosthesis orthe alignment mark provides a position reference for determining theimplanted position of the prosthesis in the cavity.

According to an embodiment of the present invention there is provided amethod of implanting a prosthesis comprising: inserting a prosthesisinto a bone cavity, the prosthesis comprising a substrate and a coatingapplied to a surface of the substrate to promote osseointegration, thecoating having at least one interruption forming an alignment mark; andaligning the interruption in the coating with a local bone feature byrotating the prosthesis within the bone cavity such that the position ofthe interruption in the coating relative to the bone cavity isindicative of the angle of insertion of the prosthesis or the portion ofthe prosthesis extending from the cavity.

The present invention will now be described, by way of example only,with reference to the accompanying drawings, in which:

FIG. 1 illustrates an acetabular cup in accordance with a firstembodiment of the present invention;

FIG. 2 illustrates in partial cross section the acetabular cup of FIG. 1implanted into a reamed bone cavity; and

FIG. 3 illustrates an acetabular cup in accordance with a secondembodiment of the present invention.

Referring first to FIG. 1, this illustrates an acetabular cup comprisinga hollow, generally hemispherical metal substrate 2 and a coating 4 overa portion of the convex surface 6, which when implanted is at leastpartially in contact with the bone. The coating 4 is intended to promoteosseointegration and may be a porous coating such as Porocoat or abioactive coating such as hydroxyapatite or any other similar coatingmaterial applicable to a surface of a prosthesis. The cup may haveadditional bone fixation means, for instance a screw hole (not shown)generally positioned at the pole of the hemisphere. The coating 4extends over the majority of the convex surface 6, but is interrupted byan uncoated portion 8 which extends from one edge of the cup rim 10. Theuncoated portion 8 is separated from the coating along a dividing line12 which extends along an arc across the convex surface 6. The arc maybegin and end at discrete points about the rim 10, or may extend to andfrom a single point on the rim 10. Alternatively, the dividing line 12may generally comprise a circle extending about the cup.

Referring now to FIG. 2, this illustrates in partial cross section thecup of FIG. 1 during insertion into a bone cavity 14 formed within apatient's acetabulum. It can be seen that the dividing line 12 extendsacross the convex surface 6 at approximately 20° to the rim 10. The cupis positioned such that the dividing line 12 is approximately alignedwith the acetabular rim 16 such that the uncoated portion 8 of the cupprotrudes from the reamed acetabular cavity or the natural acetabulum14. The angle of the dividing line 12 relative to the rim 10 is chosento allow the surgeon to clearly identify the proportion of the cup whichshould protrude from the cavity 14. It will be appreciated that in otherembodiments of the invention the angle between the dividing line 12 andthe rim 10 may vary.

The difference between the coating 4 and the uncoated portion 8 isclearly identifiable by the surgeon as both a difference in surfaceroughness and a step change in the surface of the cup (equal to thethickness of the coating 4). Consequently the uncoated portion 8 whichserves as an alignment mark for positioning the cup relative to localbone landmarks (that is, the acetabular rim 16) is clearly identifiableeven if obscured by blood or other fluids. Clear identification of thealignment marks is important in ensuring that they are readilyidentifiable during surgery. As such, forming alignment marks usingdifferential coating techniques is preferable to alignment marks formedby laser marking or applying colours to the prosthesis surface, whichmay be more easily obscured.

The cup may be readily inserted into a prepared cavity by firstlyrotating the cup about its polar axis such that the uncoated portionpoints to the edge of the cavity from which the cup is intended toprotrude and then tilting the cup until the dividing line is parallel tothe acetabular rim. As the uncoated portion of the cup is not in contactwith the bone, there is no loss of strength due to the reduction in thearea of the coating.

Referring now to FIG. 3, this illustrates an acetabular cup inaccordance with a second embodiment of the present invention. Featuresthat are common to FIGS. 1 and 2 are referred to using the samereference numbers. While FIGS. 1 and 2 illustrate a cup intended to beinserted into a bone cavity such that a fixed portion of the cupprotrudes from the cavity at a fixed angle, it may be desirable for theangle of protrusion to be chosen by the surgeon intraoperatively. Inplace of a single uncoated portion of the convex surface 6, FIG. 3illustrates a series of lines 20 within the coating 4 formed fromregions where the coating is absent from the cup. Lines 20 compriseelongate gaps within the coating such that the coating is presentbetween the lines 20. There may be four lines 20 as illustrated spacedapart from the rim 10 by 5°, 10°, 15° and 20°, however the number oflines and their spacing may vary, for instance there may only be asingle line at the same angular position relative to the rim as thedividing line 12 illustrated in FIG. 1. The lines may extend to the rimof the cup as shown in FIG. 3, however they may alternatively stop shortof the rim or not intersect the rim.

Advantageously, if only a limited angular protrusion from the cavity isrequired, the reduction of coating area represented by the lines 20 thatare positioned within the bone cavity is minimal. Conversely, for thecup of FIGS. 1 and 2, if the surgeon decides to implant the cup at asmaller angular protrusion than that indicated by the full extent of theuncoated portion then there may be a significant reduction in coating incontact with the bone compared with a cup according to the prior art forwhich the whole of the convex surface is coated.

It will be appreciated that in alternative embodiments of the presentinvention differing sizes and shapes of the convex surface may be leftuncoated to serve as alignment markings. For instance, the uncoatedlines may be replaced by dashed lines, short lines not extending fullyto the rim or only a single small circular interruption in the coating.More generally, any interruption in the coating capable of conveyingalignment information for a prosthesis relative to local bone landmarksis within the scope of the present invention.

The interruptions in the coating may be provided by masking off portionsof the prosthesis surface prior to applying the coating, as will be wellknown to the skilled person according to the particular coatingmaterial.

Although the present invention has been described above primarily withreference to a prosthetic acetabular cup, the invention is not limitedto this application. For instance, the present invention may be appliedto a cup arranged to be couple to a glenoid of a shoulder joint. Moregenerally, the present invention is applicable to any prostheticcomponent and is particular suited to applications where the prostheticcomponent may be implanted at a variable angle relative to the bone, orwith a varying proportion of the prosthesis protruding from a bonecavity, such that the interruptions in the coating assist in correctlyaligning the prosthetic component. In particular, the coatinginterruptions assist in determining the proportion of the prostheticcomponent which is intended to protrude from a bone cavity.

The interruptions in the coating have been described above as adifference between a portion of the prosthesis surface where the coatingis applied and a portion of the prosthesis surface where the coating isabsent. However, in alternative embodiments of the present invention theinterruptions may comprise a difference in coating thickness or thepresence or absence of an additional layer of coating material. Infurther embodiments of the present invention there may be no coatingmaterial applied to the surface of the substrate. Instead, selectedportions of the substrate may be treated to promote osseointegration,for instance by increasing the porosity or surface area of the material.For instance, selected portions may be grit blasted or otherwise treatedto increase their roughness. The alignment mark comprises the interfacebetween adjacent surface areas which have been differently treated topromote osseointegration to different extents. For instance, one areamay not be treated at all, or treated to a lower extent or treateddifferently.

Further modifications to, and applications of, the present inventionwill be readily apparent to the appropriately skilled person withoutdeparting from the scope of the appended claims.

The invention claimed is:
 1. A method of manufacturing a cup arranged tobe inserted into a bone cavity to form a socket component of aprosthesis ball and socket joint, the method comprising the steps of:providing a substrate having a convex surface, a rim and an apex;identifying at least a first interruption and a second interruption inthe convex surface, the first interruption and the second interruptionextending from the rim and at spaced apart locations at predeterminedangles, the predetermined angles being greater than zero; and treatingat least a first area defined as the area between the rim and the secondinterruption and including the apex to promote osseointegration;treating a second area defined as the area between the rim and the firstinterruption to promote osseointegration to a lesser extent as comparedwith the treatment of the first area.
 2. The method of claim 1, whereinsaid step of treating at least the first area comprises the steps of:masking the interruptions; and applying a coating to promoteosseointegration to the surface of the first area.